Tubular members used for well pipe such as casing and the like come in a plurality of weights for each given pipe size thus providing multiple wall thicknesses and variable maximum internal pipe diameters within each pipe weight and size. The manufacturer of such pipe desires, from an economic view point, that the pipe wall be as thin as possible while still complying with various API standards for each particular weight and size pipe, one of such being that the pipe must have a minimum internal diameter that permits a drift bar or "rabbit" to pass therethrough. This is referred to as the "drift diameter" of that particular size and weight of pipe, such as casing and is generally slightly larger in diameter than the drift bar to assure passage of the drift bar therethrough. The "drift diameter" is that diameter of a tubular member which will pass an object of predetermined length and diameter through the pipe or tubular member from end-to-end.
It is not unusual for the internal diameter of pipe within any weight and size to vary from "drift diameter" to the maximum permissible internal pipe diameter throughout its length.
Of course, the manufacturer must still meet the other requirements such as, for example, concentricity, outer diameter, inner diameter and pressure requirements of the tubular member.
The scraping of well pipe, particularly such as casing, has heretofore been effected by a plurality of members supported on a body for radial movement in an attempt to scrape the pipe even though there are variations in the pipe internal diameter. The device is positioned in the well pipe and rotated and/or reciprocated to attempt to remove cement sheaths, burrs, mill scale, and other objects from the interior wall of the pipe so as to prevent damage to packers and other devices that may be used in the completion and production of the well.
Although such prior devices employ members that are radially contractible and expansible, the scraping surfaces on the scraper members or blades wear. Thus, there is no assurance that after the device has been used one or more times, scraping of the well pipe is effected from the minimum internal drift diameter to the full internal diameter as there is no way to assure that the scraper surfaces on the blades will always project outwardly into contact with the maximum internal diameter of the pipe. Thus, such devices must rely on the wobble, bounce or other kind of movement of the scraper tool as it is reciprocated or rotated in the well pipe in the hope that the scraper surfaces will contact and effect scraping of the interior pipe wall. As a result packers or other devices lowered into the well may be damaged or incapable of holding well pressure or being properly positioned by debris which is not removed and remains on the interior pipe wall.
More recently in an endeavor to overcome this problem a device employing scraper blades that are adjustable radially to a fixed location on a tapered surface has been employed, but such arrangement does not permit the scraper blade to expand from the fixed position, and the initial fixed position of the scraper blade can, generally speaking, be maintained approximately only at drift diameter since if it is larger than drift diameter there is no assurance that it can be lowered into and manipulated in the pipe. Thus, while such arrangement may contract and scrape the minimum internal diameter of the pipe, that is, the drift diameter of a pipe, the maximum internal diameter of the pipe is not necessarily engaged and scraped by the fixed blade.
Other problems with prior art scrapers such as those above mentioned include the fact that the construction provides an arrangement so there is substantially a line contact between the scraping surfaces and the interior pipe wall at its minimum internal diameter (drift diameter) as well as at its maximum internal pipe diameter since the radius of curvature at each diameter of the pipe is different, and may also be substantially different from the radius of curvature on the cutter surfaces.
Generally speaking, as a practical matter, a scraper blade manufacturer must manufacture scraper blades that cover ranges of pipe weights and therefore as the scraper surfaces on the blade wear the scraper surfaces may not contact and scrape lighter weight pipe in each size which reduces the effective range of pipe weights that can be effectively scraped by the scraper blades.
When the scraper surfaces on scraper blades will no longer scrape the heavier weight pipe in each pipe size, it is considered worn out, even though there may be substantial scraper surface depth remaining on the blade, and is either rebuilt or replaced with new pipe scraper blades.